A motor vehicle steering system is configured, as shown in FIG. 24, so that a rotation of a steering wheel 1 is transmitted to an input shaft 3 of a steering gear unit 2, and a pair of left and right tie-rods 4, 4 are pushed or pulled in association with a rotation of the input shaft 3, whereby a turning angle is given to front wheels. The steering wheel 1 is fixedly supported at a rear end portion of a steering shaft 5, and this steering shaft 5 is supported rotatably in a cylindrical steering column 6 while being inserted through the steering column 6 in an axial direction. A front end portion of the steering shaft 5 is connected to a rear end portion of an intermediate shaft 8 via a universal joint 7, and a front end portion of this intermediate shaft 8 is connected to the input shaft 3 via another universal joint 9.
In steering systems like the one described above, electric power steering systems have conventionally been known which includes an electric assisting device in which an electric motor 10 is used as an auxiliary power source for realizing a reduction in force that is used to operate the steering wheel 1. A reduction gear is incorporated in such an electric power steering system. As this reduction gear, a worm reduction gear has conventionally been known as described in Patent Document 1 or the like and is used generally. This worm reduction gear has a large lead angle and is reversible in relation to a power transmitting direction. FIG. 25 shows an example of a worm reduction gear having a conventional construction that is described in Patent Document 1. The worm reduction gear 11 includes a housing 12, a worm wheel 13 and a worm 14.
The housing 12 is fixedly supported on the electric motor 10 and has a wheel accommodating portion 15 and a worm accommodating portion 16 that exists in a skew position relative to the wheel accommodating portion 15 and which is brought into continuous connection with the wheel accommodating portion 15 at an axially intermediate portion thereof. The worm wheel 13 is fixedly supported on a steering shaft 5 (refer to FIG. 24) concentrically therewith at a portion that lies close to a front end portion thereof, the steering shaft 5 being a driven shaft and being supported rotatably inside the wheel accommodating portion 15. The worm 14 is supported rotatably inside the worm accommodating portion 16 by a pair of rolling bearings 18a, 18b such as deep groove ball bearings at two axial positions that hold therebetween worm teeth 17 that are provided on an axially intermediate portion of the worm 14 with the worm teeth 17 made to mesh with the worm wheel 13. An output shaft, which is a drive shaft, of the electric motor 10 is connected to a proximal end portion (a left end portion in FIG. 25) of the worm 14. Namely, this worm 14 can be driven to rotate by the electric motor 10.
In the case of the worm reduction gear 11 that has conventionally been in general use as described above, an inevitable backlash exits at a meshing portion between the worm wheel 13 and the worm teeth 17 based on a tolerance on dimension of or a tolerance on assemblage of constituent members of the worm reduction gear 11. The existence of such a backlash will possibly generate harsh gear tooth striking noise at the meshing portion in changing the rotational direction of the steering shaft 5. Then, in the case of the example of the conventional construction described in Patent Document 1, the proximal end portion of the worm 14 is supported on the worm accommodating portion 16 so that the proximal end portion can oscillate slightly. Since this oscillating angle is slight, the oscillation can easily be absorbed by the use of a bearing having small moment rigidity such as a single-row deep groove ball bearing. A pressing dowel 19 is fitted on a distal end portion (a right end portion in FIG. 25) of the worm 14, and a coil spring 20 is provided between the pressing dowel 19 and the worm accommodating portion 16. Then, the distal end portion of the worm 14 is pressed radially (upwardly in FIG. 25) towards the worm wheel 13 by the coil spring 20 via the pressing dowel 19. By adopting this configuration, the backlash between the worm teeth 17 and the worm wheel 13 is suppressed, whereby the generation of the gear tooth striking noise is suppressed.
In the case of the conventional construction described above, however, there is room for improvement in relation to what will be described below.
Namely, an annular gap is provided between an outer circumferential surface of the pressing dowel 19 that is fitted on the distal end portion of the worm 14 so as to press radially the distal end portion of the work 14 towards the worm wheel 13 and an inner circumferential surface of the worm accommodating portion 16 so as to extend fully circumferentially. Consequently, when the rotational direction of the steering shaft 5 is changed, the distal end portion of the worm 14 is displaced in a direction (a front-to-rear direction of a sheet of paper on which FIG. 25 is drawn) that is at right angles to a direction in which the distal end portion of the worm 14 is pressed by the coil spring 20 to thereby possibly generate a harsh gear tooth striking noise at the meshing portion. In particular, in case the meshing position of the worm teeth 17 with the worm wheel 13 varies based on a tolerance on dimension of or a tolerance on assemblage of the constituent members of the worm reduction gear 11, the distal end portion of the worm 14 is displaced more in the direction that is at right angles to the direction in which the distal end portion of the worm 14 is pressed by the coil spring 20, leading to a possibility that the gear tooth striking noise is generated easily. In addition, in case the meshing position is varies, the friction loss of the meshing portion becomes great, and this leads to a possibility that the magnitude of auxiliary power (torque) that is exerted on the steering shaft 5 from the electric motor 10 changes (decreases) by such an extent that the friction loss becomes great.
The variation of the meshing position of the worm teeth 17 with the worm wheel 13 becomes remarkable when the tooth flanks of the worm teeth 17 and the worm wheel 13 wear as the worm reduction gear 11 operates. The worm wheel 13 may be fabricated by joining fixedly a synthetic resin tooth portion to a circumference of a metallic circular disc member so as not only to suppress the generation of gear tooth striking noise or sliding noise at the meshing portion but also to realize a reduction in weight of the worm wheel 13 and hence the worm reduction gear 11, as the tooth flanks of the worm wheel 13 tend to wear easily in association with the operation of the worm reduction rear 11, which facilitates the increase in variation of the meshing portion.